Metal-ceramic fitting assembly

ABSTRACT

A metal-ceramic composite fitting assembly is described in which an effective connection between a boss on the ceramic member and the metal shaft member is achieved by providing the shaft with a sleeve formed of a material having a coefficient of thermal elongation comparable to that of the ceramic member, which sleeve is joined to the shaft member and cooperates with a recess therein to engage the boss along its length and to impose thereon compressive gripping forces that increase in magnitude along the length of the boss.

BACKGROUND OF THE INVENTION

The general field of the present invention is interconnection of ceramicmembers to metallic members.

Currently ceramics are increasingly being used in place of metals inmechanical environments where heat resistance and/or low specificgravities are important. For example, ceramic turbine disks arereplacing metal disks in the turbo-superchargers of internal combustionengines. The turbo-supercharger is a unit for pre-compressing intake airor air/fuel mixture. The supercharger makes use of the pressuregenerated by exhaust gases to turn an exhaust turbine disk which drivesa compressor. Since the turbine disk is a member which is exposed to hotexhaust gases and which rotates at high speeds, it is more efficient tomake the disk of ceramics which have higher heat resistance and lowerspecific gravities than metals. Moreover, certain ceramics are as strongas metals.

On the other hand, metals are still used for certain purposes andtherefore a means of joining the ceramic parts to the metal parts mustbe found. The problem is the thermal expansion coefficients of themetals used tend to be two to five times higher than those of ceramics.For example, Cr-Mo steel has a coefficient of thermal expansion ofE=11.7×10⁻⁶ /°C. whereas the ceramic, silicon nitride (Si₃ N₄), has acoefficient of thermal expansion of E=2.6 to 4.5×10⁻⁶ /°C. In hightemperature environments, metal parts thus have a tendency to expandmore rapidly than the ceramic parts, often causing the metal to pullaway from or break the ceramic.

Various means for joining ceramics to metals have been proposed.Examples are described in Japanese Patent Application Laid-Open No.103902/1984 and Japanese Utility Model Application Laid-Open No.5701/1984, the disclosures of which are incorporated herein by referenceand are briefly discussed in the Detailed Descriptions of the Drawingsof this invention. In the above disclosures, a boss portion of a ceramicdisk extends into a cup-shaped end of a metal shaft. The boss portion isjoined by a shrink-fit connection or by brazing to the sleeves of thecup-shaped end which sleeves are made of metal with a thermal expansioncoefficient that is substatially the same as that of the ceramic. Sincethe ceramic boss and the metallic sleeve expand at a nearly equal rate,normally the connection between the boss and the sleeve is not broken athigh temperatures. However, other cracks and failures can occur in theseprior art arrangements.

SUMMARY OF THE INVENTION

The present invention relates to composite ceramic metal assemblies someof which may have solder in contact with the ceramic and the metalmember for adhesion, at least some of the metal having a substantiallyhigher coefficient of thermal expansion than the ceramic. According toone aspect of the invention the boss portion of a ceramic disk isinsertedinto a cup-shaped end of a metallic shaft and is heated to atemperature much higher than the melting point of the solder. Liquidsolder thereby penetrates between the ceramic boss and the metalliccup-shaped end while the temperature remains high. The entire assemblyis then allowed to cool. Because the thermal expansion coefficient ofthe metallic cup-shaped end is much higher than that of the ceramicboss, the space between the cup-shaped end and the boss is greater thanit would be were the solder injected when the assembly was cool.Therefore, there is more solder in the space at the higher temperatureand as the assembly cools the metallic end contracts against thesolidified solder thus gripping the ceramic boss.

According to another aspect of the invention the connection between thecup-shaped end of the member and the ceramic boss can be effectedwithout solder by providing the interior surface of the cup-shaped endwith an inwardly converging taper such that when the metal-member isshrunk fit onto the boss an effective gripping action occurs.

Accordingly, the present invention has the advantage of using a metal ofa higher coefficient of thermal expansion than a ceramic to grip theceramic. Other and further advantages will appear hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view showing the prior art fitting assemblyof Japanese Utility Model Application Laid-Open No. Utility Model5701/1984.

FIG. 2 is a sectional side view showing the prior art fitting assemblyof Japanese Patent Application Laid-Open No. 103902/1984.

FIG. 3 is a sectional side view showing one embodiment of the presentinvention.

FIG. 4 is a top view of a ceramic disk taken along line IV--IV of FIG.3.

FIG. 5 is an exploded assembly sectional side view of the fittingassembly of FIG. 3.

FIG. 6 is a schematic sectional side view of a part of the fittingassembly of FIG. 3 illustrating the heating and expanded state duringthe soldering operation.

FIG. 7 is a schematic sectional side view of a part of the fittingassembly of FIG. 3 illustrating the cooled and shrunken state after thesoldering operation.

FIG. 8 is a sectional side view showing essential portions of analternate embodiment of the present invention.

FIG. 9 is a schematic sectional side view showing the fittingrelationships of the interconnected metal shaft and ceramic body of thepresent invention.

FIG. 10 is a graph depicting test results of the embodiment of FIG. 3.

DETAILED DESCRIPTION OF THE DRAWINGS

In the disclosure of the Japanese Utility Model Application Laid-OpenNo. 5701/1984, as shown in FIG. 1, there is connected to a boss 02 of aceramic disk 01 by the shrinking or brazing method a sleeve 03, which ismade of a metal having a coefficient of thermal expansion that issubstantially the same as that of the ceramic. The sleeve 03 is weldedto a metallic shaft 04. As a result, stress occurs at the welded portionof the sleeve 03 and the shaft 04 because the coefficient of thermalexpansion of the shaft 04 is much higher than that of the sleeve 03. Thestress tends to be concentrated in the bottom corner 03a of the sleeve03 and in the corner 04a of the shaft 04. This makes cracks apt to occuralong a line, indicated as 05, joining the two corners 03a and 04a. If alarge shrinking allowance is made when the sleeve 03 is shrunk onto theboss 02, the boss 02 tends to rupture because of the high stressgenerated at the root thereof. If the shrinking allowance is small, theadhesion force acting upon the contact portion between the boss 02 andthe sleeve 03 is uniformly distributed so that the boss 02 tends to comeout of the sleeve 03.

In the disclosure of Japanese Patent Application Laid-Open No.103902/1984, as shown in FIG. 2 and wherein like numerals are employedto designate elements corresponding to those shown in FIG. 1, there isfitted on the circumference of the boss 02 of the ceramic disk 01 acylindrical sleeve 06, which has a coefficient of thermal expansionapproximately equal to that of the ceramic disk 01, and has its end face06a fixed to the metallic shaft 04 by welding. In the device, a crack 07is liable to be formed along the joined faces of the cylindrical sleeve06 and the metallic shaft 04 in their thermally loaded state as a resultof the difference between the coefficients of thermal expansion of thesleeve 06 and the shaft 04. Also, the shaft-end cylindrical portion 04bof the metallic shaft 04, which is adjacent to the end face 06a of thecylindrical sleeve 06, is affected by the aforementioned difference inthe coefficients of thermal expansion. Because the sleeve 06 and the endportion 04b are welded together, the end portion 04b tends to resist thethermal expansion of the shaft portion 04c of the metallic shaft 04 sothat a crack 08 is liable to be made along the line joining the metallicshaft end face 04d and the metallic shaft corner 04a. Moreover, the sameshrinkage problems that were discussed in the foregoing paragraph withregard to the FIG. 1 device occur in this device thus tending to causethe boss 02 to either rupture or come out of the sleeve 06. Further, itshould be noted that the end face of the boss 02 is not secured to themetal member in either embodiment of the prior art, FIGS. 1 and 2, butrather is spaced from the facing end of the metal member to be grippedonly by the inner surface of the extending sleeve.

The present invention will now be described with reference to the firstembodiment shown in FIGS. 3 to 7 as applied to a turbo-superchargerattached to an automotive internal combustion engine. Theturbo-supercharger is a unit for compressing intake air or air/fuelmixture. The supercharger uses the pressure generated by exhaust gasesto turn an exhaust turbine disk which drives a compressor by aconnecting metal shaft.

As illustrated in FIGS. 3 and 4, a turbo-supercharger rotor isconstructed of a composite assembly including a turbine disk 1 made of,for example, silicon nitride ceramics, a sleeve 10 made of a metal alloy(for example, 23 to 30 wt. % of Ni, 17 to 30 wt. % of Co, 0.6 to 0.8 wt.% of Mn and the remainder of Fe) having a coefficient of thermalexpansion substantially equal to that of the silicon nitride ceramics, arotary shaft 20 made of Cr-Mo steel, and a compressor drum (not shown)assembled integrally with the rotary shaft 20. The compressor drum (notshown) is fitted on the smaller-diameter portion 29 of the rotary shaft20 and is assembled integrally with the same by means of a nut (notshown) which is fastened on a male thread 30 on the shaft 20.

Turning to FIG. 5, the sleeve 10 has its inner circumference sized andshaped to be fitted with a slight but generally constant gap on a boss 3formed on the turbine disk 1. This gap extends from the boss-rootcircumference 5 to the boss-end circumference 7. The rotary shaft 20 hasan end portion 21 formed with a larger diameter than that of its centralportion 27. The end portion 21 is provided with a cylindrical recess 22that has an inner surface whose diameter is substantially the same asthe inner diameter 11 of the sleeve 10 which provides a slight butsubstantially constant gap between it and the boss end portion 6 of theaforementioned turbine disk 1. Communication between the end so-formedcup-shaped portion 21 and the central portion 27 of the shaft 20 isprovided through a communication hole 28 which extends between thebottom 23 of the cylindrical recess 22 and the outer circumference ofthe central portion 27 as shown in FIG. 3.

Turning now to a description of the steps in the assembly process asshown in FIGS. 5, 6 and 7. The ceramic turbine disk 1, the sleeve 10 andthe rotary shaft 20, can be assembled together in the following order.The end face 14 of the sleeve 10 and the leading end face 24 of the endportion 21 of the rotary shaft 20 are integrally joined by, for example,a frictional welding operation to extend the effective length of thecup-shaped end of the shaft. The recess bottom 23 and the communicationhole 28 of the rotary shaft 20 are then filled with a soldering material31, and the boss 3 of the ceramic turbine disk 1 is positioned withinthe inner circumference 11 of the sleeve 10 and in the cylindricalrecess 22 of the rotary shaft 20. In this state, the assembled membersare heated together to a temperature higher than the melting temperature(for example, 700° C.) of the soldering material 31. Since the metalmaterial of the rotary shaft 20 has a coefficient of thermal expansionlarger than the materials of the ceramic turbine disk 1 and the sleeve10, their unequal expansion causes the diameter between the innercircumference 11 of the sleeve 10 and the circumference of thecylindrical recess 22 of the rotary shaft 20 to increase gradually inthe direction of the recess bottom 23 (as shown in FIG. 6). The gap 32which is thus formed is filled with the molten soldering material 31without any clearance.

When the temperature of the assembly of the ceramic turbine disk 1, thesleeve 10 and the rotary shaft 20 is gradually dropped to a level lowerthan the melting temperature (at about 700° C.) of the solderingmaterial 31, the soldering material starts to solidify before the gap 32is restored to its normal state at room temperature (as shown in FIG.7). When the temperature of the assembly reaches room temperature, therotary shaft 20 shrinks more than the ceramic turbine disk 1 and thesleeve 10 since the rotary shaft has the largest coefficient of thermalexpansion. The thicker portion of the soldering material 31 present atthe inner end of the assembled sleeve 10 and recess portion 22 resiststhe shrinkage of the recess portion so that the boss 3 of the siliconnitride ceramic turbine disk 1 is subjected (as shown by the arrows inFIG. 7) to a fastening force which increases gradually from its root 4to its leading end 6.

Accordingly boss 3 of the ceramic turbine disk 1 is grasped not only bythe sleeve 10 that is bonded to the end portion 21 of the rotary shaft20 but also by the cylindrical base portion or recess 22 of the rotaryshaft 20, and the securing force is increased gradually along the axiallength of the boss. The solder covers the entire boss 3, both on thecylindrical surface and on the flat end. All of this makes it possibleto prevent the ceramic turbine disk 1 from becoming disconnected fromthe steel rotary shaft 20. Moreover, on the boss 3 of the ceramicturbine disk 1, the fastening force increases gradually from theboss-root 4 close to the disk body 2 toward the boss end 6 so that nostrong stress is generated in the boss root circumference 5 to preventthe boss 3 from fracturing and separating from the disk body 2.Moreover, since the boss root circumference 5 is gently curved, as shownto have a large radius of curvature, the stress tends not to beconcentrated in the vicinity of the boss root circumference 5.

As illustrated in FIG. 5 the end portion 21 of the rotary shaft 20 isfurthermore, preferably formed with a shaft end portions 25 and 26 thatare gently curved. As a result of this configuration, the stress isuniformly distributed about the end portion 21 rather than beingconcentrated at a particular point so that a crack is much less likelyto occur between the corner of the cylindrical recess 22 and the shaftportions 25 or 26.

In the embodiment shown in FIGS. 3 to 7, the boss 3 of the ceramicturbine disk 1 is joined to both the sleeve 10 and the cylindricalrecess 22 of the rotary shaft 20. Alternatively, as shown in FIG. 8, theboss 3 may be integrally joined to the sleeve 10 and the cylindricalrecess 22 without the use of soldering material, that is solely orprimarily by only the predesigned shrinkage. In this modification, theinner circumference 11 of the sleeve 10 and the cylindrical recess 22 ofthe rotary shaft 20 is slightly tapered in their room temperature statewhich taper converges toward the recess bottom 23, to provide theaftermentioned desired gripping force configuration on the boss 3following heating of the shaft for inserting the boss and subsequentcooling.

FIG. 9 schematically illustrates the state in which a rotary shaft 20and a ceramic disk 1 such as disclosed with regard to FIGS. 3 to 7 arejoined integrally by fitting the ceramic boss 3 of the ceramic disk 1 inthe sleeve 110 of the rotary shaft 20 and by using a soldering material31 sandwiched between their fitting and fitted faces. The ceramic boss 3of the ceramic disk 1 is inserted into the cylindrical fitting hole ofthe rotary shaft 20, and these two members are heated to a temperaturehigher than the melting point of a soldering metal 31 so that thesoldering material 31 may penetrate the gap provided between the bossand the fitting hole. After being cooled down to room temperature, theceramic boss 3 and the rotary shaft 20 in the soldered state aresubjected in the axial direction between portions A and B to an internalstress expressed by L(E1-E2)t (where E1 and E2 designate thecoefficients of thermal expansion of the rotary shaft 20 and the ceramicdisk 1, "t" designates the difference between the soldering temperatureand the room temperature and L is the dimension indicated in FIG. 9).Because of the large shrinkage of the sleeve 110 during cooling, acompressive stress is generated in the surface layer of the ceramic boss3 between points A and B so that a tensile stress is generated in theportion A. As a result, an excessive fitting length L will cause arupture in the portion A of the ceramic boss 3 in combination with thebending stress which is generated in the ceramic boss 3 of the ceramicdisk 1, when the two members 1 and 3 are rotated. Moreover, since theceramic disk 1 has its total length increased, large vibrations aregenerated thus causing more stress in the joined portions. On the otherhand, an insufficient fitting length L will result in an insufficientarea of contact for the soldering of the joining faces so that theceramic disk 1 tends to come out of the sleeve 110.

After conducting experiments, the present inventors have found that themost satisfactory result can be attained by making the fitting length Land the hole diameter D satisfy the equation 0.4≦L/D≦1.0. In this case,the ceramic disk 1 can be used for a long term without rupturing orcoming out of the metallic shaft member.

The assembly having the disk 1 and the rotary shaft 20 combined asdescribed above was tested in eleven different relative sizes or ratiosas shown by points in the graph of FIG. 10 and listed in the followingTable 1, the fitting length L to the internal diameter D of the sleeve10. These samples were rotated at high speeds on the rotary shaft 20 andexamined as to whether or not troubles were caused until the target oroperating r.p.m., No, was reached.

                  TABLE 1                                                         ______________________________________                                        Samples                                                                              1. D    2. L   3. L/D 4. R.P.M.                                                                            5. Remarks                                ______________________________________                                        1      9.7     7.3    0.75        No  No Trouble                              2      9.7     15.7   1.62   0.70 No  Ceramics Bent                           3      12      3.3    0.28   0.76 No  Ceramics                                                                      Disjointed                              4      12      3.3    0.28   0.72 No  Ceramics                                                                      Disjointed                              5      12      5.3    0.44        No  No Trouble                              6      12      7.3    0.61        No  No Trouble                              7      12      7.3    0.61        No  No Trouble                              8      12      12.3   1.03        No  No Trouble                              9      12      12.3   1.03   0.89 No  Ceramics Bent                           10     13      14.3   1.10        No  No Trouble                              11     13      14.3   1.10   0.77 No  Ceramics Bent                           ______________________________________                                    

In Table 1, the third column lists the L/D values of the respectivesamples, the fifth column the contents of the troubles caused, and thefourth column the r.p.m. as a decimal of No in the case of problems andthe target r.p.m., No, in the case of no trouble. FIG. 10 is a graphplotting the relationship between the values listed in theaforementioned third and fourth columns, i.e., the ratio L/D and ther.p.m.

For the ratio L/D ranging from 0.4 to 1.03, as is apparent from FIG. 10,no trouble arises even if the rotations of the samples reach the targetr.p.m., No. Outside of the range (i.e., L/D<0.4 or L/D>1.03), thetroubles listed in the fifth columns of the Table 1 occurred before thetested sample reach the target r.p.m., No. From this it may be seen thatthe proper ratio of length to diameter of the cup-shaped end producesthe desired gripping of the ceramic boss 3 for optimum performance.

What is claimed is:
 1. A metal-ceramic composite body including a memberformed of ceramic material having a generally cylindrical boss extendingfrom the surface of said member to a free leading end and a shaft formedof a metal material whose coefficient of thermal expansion is greaterthan that of said ceramic material, and means for connecting said shaftto said boss, comprising:a cup-shaped end on said shaft defined by arecess on said shaft receiving the leading end of said boss and a sleeveaffixed to said shaft end, said sleeve having a through-openingcoaxially aligned with said recess in said shaft to enclose said bossand being formed of a metal whose coefficient of thermal expansioncorresponds substantially with that of said ceramic material, andbearing means interposed between facing surfaces of said connectingmeans and said boss for imposing a compressive force about the surfaceof said boss that increases in magnitude axially along said boss towardthe leading end thereof upon heating said composite body and subsequentcooling thereof.
 2. The metal-ceramic composite body as recited in claim1 in which said bearing means comprises a taper formed on alignedsurfaces of said sleeve and said recess, said taper converging towardthe end of said recess.
 3. The metal-ceramic composite body as recitedin claim 1 in which said bearing means comprises a body of soldermaterial between the exterior of said boss and the aligned surfaces ofsaid sleeve and said recess.
 4. The metal-ceramic composite body asrecited in claim 3 in which the thickness of said body of soldermaterial increases axially along said boss prior to cooling of saidcomposite body.
 5. The metal-ceramic composite body as recited in claim4 in which said solder material is fillingly disposed between said bossand said cup-shaped end of said shaft.
 6. The metal-ceramic compositebody as recited in claim 5 in which the aligned surfaces of said sleeveopening and said recess define an opening receiving said boss having alength-to-diameter ratio of between about 0.4 to about 1.0.